2 Answers
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At beer pressures, a keg cannot explode. It's designed to take much more pressure - rated to around 120-130 psi. Even at failure, the seals will fail rather than the chamber itself. Failing at standard beer pressures will be as a leak (pinhole or crack).

Which isn't to say they can't explode. It's a sealed pressurized vessel - so it could explode or implode. It's just not normal for beer to rapidly reach the pressure required to cause an explosion (or for outside pressure to cause an implosion).

Just to be clear this is regarding steel or aluminium kegs... in the end they'll all fail at a high enough pressure but will they explode or will they leak first (not everything explodes)? In mechanics terms are they a thin walled or thick walled vessel. The thicker the wall is the stronger it could be but also longer the cracks are that can grow in it as the pressure rises (metals naturally full of tiny cracks which all start to grow as the pressure rises in the keg) and eventually one crack will reach the critical crack length when it grows very quickly, at the speed of sound in the metal, this is what leads to an explosion. So a wise pressure vessel designer keeps the material as thin as possible (whilst being strong enough to do the job) so the crack will run from inner to outer surface before it reaches the critical crack length and start to release the pressure, in this case you'll get "Leak before Burst" failures. I should have added, liquids are more of less incompressible so can't store any energy, so the more gas that's involved the more violent an explosion is likely to be as all that stored energy is released. SO if you ever do want to test to destruction, fill the keg with as much water as possible to keep the amounts of gas to a minimum, then you can achieve the same pressures without suddenly releasing lost of energy.